We will soon be able to resolve Mars microbes from orbit. ;-)

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Robert Clark

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Jan 10, 2007, 12:12:09 PM1/10/07
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On another space oriented forum I noted:

"It took 20 years to increase the resolution by a factor or 10 over
Viking with the Mars Global Surveyor mission. But only 10 years to
increase the resolution over that of MGS by a factor of 10 with Mars
Reconnassance Orbiter.
Could we increase the resolution over MRO by another factor of 10 to,
gulp, 3 cm per pixel in only 5 years this time?"

Funny though, that rather off-the-cuff estimate of mine is close to
what is possible.
To resolve 3 cm in the optical from say a 300 km orbit would require a
6 meter mirror. The James Webb Space Telescope will have a 6.5 meter
mirror and is scheduled for launch in 2013. But it was originally
scheduled for launch in 2011:

James Webb Space Telescope.
http://en.wikipedia.org/wiki/James_Webb_Space_Telescope

So going by this rate, it'll be 3mm/pixel 2.5 years after that, and
300 microns 1.25 years after that, and ...
Hmm, in less than a decade then we should be able to resolve microbes
from space.

Admittedly though, the JWST is a 4 billion dollar mission. Also it
uses a beryllium metal mirror for infrared astronomy only. The
beryllium makes the mirror lightweight but it is unclear if you can
achieve the much more stringent smoothness requirements at optical
wavelengths with a metal mirror.
As for the data storage and transmission of the large files for such
high resolution images, data storage capacity and costs are doubling
and halving each year, respectively:

Bye-bye hard drive, hello flash.
By Michael Kanellos
Staff Writer, CNET News.com
Published: January 4, 2006, 10:00 AM PST
"Currently, NAND chips double in memory density every year. The
cutting-edge 4-gigabit chips of 2005, for example, will soon be
dethroned by 8-gigabit chips. (Memory chips are measured in gigabits,
or Gb, but consumer electronics manufacturers talk about how many
gigabytes, or GB, are in their products. Eight gigabits make a
gigabyte, so one 8Gb chip is the equivalent of 1GB.)
"Another driving factor in the uptake of the technology is cost: NAND
drops in price about 35 to 45 percent a year, due in part--again--to
Moore's Law and in part to the fact that many companies are bringing on
new factories."
http://news.com.com/Bye-bye+hard+drive,+hello+flash/2100-1006_3-6005849.html

MRO uses the type of flash memory chips discussed here.

Also, interestingly NASA had planned a laser communication orbiter for
Mars for launch in 2010 before it was canceled:

Record Set for Space Laser Communication.
By Ker Than
Staff Writer
posted: 05 January 2006
02:11 pm ET
http://www.space.com/missionlaunches/060104_laser_comm.html

Mars Telecommunications Orbiter: Interplanetary Broadband.
By Bill Christensen
posted: 05 May 2005
06:41 am ET
http://www.space.com/businesstechnology/technology/technovel_marstelecom_050505.html

This would have allowed data transmission rates of a hundred times
greater than what is currently available.

It was the great cost overruns overruns that led to cancelling of the
Mars Telecommunications Orbiter, and great cost overruns also
threatened JWST as well.
That the costs for computer technology are dropping exponentially with
capacity increasing exponentially is no doubt fueled by the free market
in this sphere.
Conversely, that launch costs are staying static is no doubt because
the launches are controlled by large governments. When private
companies become the primary financer and purveyor of launches, the
launch costs will also drop dramatically.


Bob Clark

Sam Wormley

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Jan 10, 2007, 12:17:56 PM1/10/07
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Robert Clark wrote:

> To resolve 3 cm in the optical from say a 300 km orbit would require a
> 6 meter mirror.

What is the effect of the Martian atmosphere on the goal of 3 cm
resolution?

John Popelish

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Jan 10, 2007, 12:31:26 PM1/10/07
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Robert Clark wrote:
> On another space oriented forum I noted:
>
> "It took 20 years to increase the resolution by a factor or 10 over
> Viking with the Mars Global Surveyor mission. But only 10 years to
> increase the resolution over that of MGS by a factor of 10 with Mars
> Reconnassance Orbiter.
> Could we increase the resolution over MRO by another factor of 10 to,
> gulp, 3 cm per pixel in only 5 years this time?"
>
> Funny though, that rather off-the-cuff estimate of mine is close to
> what is possible.
> To resolve 3 cm in the optical from say a 300 km orbit would require a
> 6 meter mirror. The James Webb Space Telescope will have a 6.5 meter
> mirror and is scheduled for launch in 2013. But it was originally
> scheduled for launch in 2011:
>
> James Webb Space Telescope.
> http://en.wikipedia.org/wiki/James_Webb_Space_Telescope
>
> So going by this rate, it'll be 3mm/pixel 2.5 years after that, and
> 300 microns 1.25 years after that, and ...
> Hmm, in less than a decade then we should be able to resolve microbes
> from space.
(snip)

I would rather the money be spent of building infrastructure
in orbit around Mars (the functional equivalent of cell
phone and GPS service, planet wide), and then drop hundreds
or thousands of small, self reliant rovers with lots of
different specialized sensors to comb the planet, get close
enough to actually find those microbes, if they exist, and
lots of other observations, and report back through the
overhead services. And this time, include on the rovers,
flea bots, that clean the lenses and solar cells of dust.

AustinMN

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Jan 10, 2007, 1:43:33 PM1/10/07
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Robert Clark wrote:
<snip>

> So going by this rate, it'll be 3mm/pixel 2.5 years after that, and
> 300 microns 1.25 years after that, and ...
> Hmm, in less than a decade then we should be able to resolve microbes
> from space.

So how long until Mars orbiters will be better able to resolve atomic
and sub-atomic scale images from orbit better than Earth-bound
instruments do it today? ;-)

Austin

dbo...@mindspring.com

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Jan 10, 2007, 4:11:18 PM1/10/07
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The required smoothness is no problem. I have a Be mirror flat that
was polished to about 10 angstrom roughness.

El Guapo

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Jan 10, 2007, 4:25:50 PM1/10/07
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"John Popelish" <jpop...@rica.net> wrote in message
news:fNWdncLVcpGBvTjY...@comcast.com...

OK, you had me until the flea bots.

Yousuf Khan

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Jan 10, 2007, 4:47:57 PM1/10/07
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Robert Clark wrote:
> On another space oriented forum I noted:
>
> "It took 20 years to increase the resolution by a factor or 10 over
> Viking with the Mars Global Surveyor mission. But only 10 years to
> increase the resolution over that of MGS by a factor of 10 with Mars
> Reconnassance Orbiter.
> Could we increase the resolution over MRO by another factor of 10 to,
> gulp, 3 cm per pixel in only 5 years this time?"
>
> Funny though, that rather off-the-cuff estimate of mine is close to
> what is possible.
> To resolve 3 cm in the optical from say a 300 km orbit would require a
> 6 meter mirror. The James Webb Space Telescope will have a 6.5 meter
> mirror and is scheduled for launch in 2013. But it was originally
> scheduled for launch in 2011:
>
> James Webb Space Telescope.
> http://en.wikipedia.org/wiki/James_Webb_Space_Telescope

Wow, from this it looks like it's going to be orbitting at the L2
Lagrangian Point, which would mean that it is stationary with respect to
the Earth and the Sun, and it will be behind the Earth on the night side
of the planet at all times, 1.5 million km's out. If it's behind the
Earth, wouldn't it automatically be shielded from the Sun's rays? Or is
the Earth's shadow not big enough out there?

Yousuf Khan

Sam Wormley

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Jan 10, 2007, 5:06:44 PM1/10/07
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The earth's moon will perturb it position.

AustinMN

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Jan 10, 2007, 5:10:15 PM1/10/07
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I can't answer your question, but from my understanding, being cold
(i.e. shielded from the sun) is a good thing when studying infrared.
Is it nuclear powered?

Austin

John Popelish

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Jan 10, 2007, 5:22:16 PM1/10/07
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Do you think they are a worse idea than sitting and waiting
for a tornado to pass directly over the panel to suck the
dust off, like Spirit and Opportunity do? Why not make a
little version of one of these to keep the solar panels
spotless?

http://www.vcdiscounter.com/01robotic.html

malibu

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Jan 10, 2007, 5:23:22 PM1/10/07
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I had a 1 GB computer with 600 Gb of memory
in my hand yesterday. It weighs about 5 pounds.
It is the size of a modem.
It is our new video producer.
Cost us $200.
Sells for $750.
Ka-ching!
John

jacob navia

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Jan 10, 2007, 5:24:36 PM1/10/07
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John Popelish a écrit :

> Do you think they are a worse idea than sitting and waiting for a
> tornado to pass directly over the panel to suck the dust off, like
> Spirit and Opportunity do? Why not make a little version of one of
> these to keep the solar panels spotless?
>
> http://www.vcdiscounter.com/01robotic.html

Thos are designed for a thick atmosphere and not a thin one,
but it is true that it should work!

Sam Wormley

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Jan 10, 2007, 5:29:10 PM1/10/07
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malibu wrote:

> I had a 1 GB computer with 600 Gb of memory


Perhaps you had have a 1 GHz computer with 600 GB of RAM in
your hand.

rick++

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Jan 10, 2007, 5:32:43 PM1/10/07
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Carl Sagan addresses this.

Life is likely to cause an atmosphere out of chemical equilibrium.
For example on Earth free oxygen is unstable without life and would
eventually combine into rocks.

Mars is slightly out equilibrium with the presence of methane
discovered a while back.

Phineas T Puddleduck

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Jan 10, 2007, 6:23:23 PM1/10/07
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In article <1168467014.9...@k58g2000hse.googlegroups.com>,
"AustinMN" <tacoo...@hotmail.com> wrote:

> I can't answer your question, but from my understanding, being cold
> (i.e. shielded from the sun) is a good thing when studying infrared.
> Is it nuclear powered?
>
> Austin

The colder the better. When you are looking for black body's emitting at
say 100K, then having a telescope at 100K is the equivalent of shining a
torch down your telescope and trying to see the moon l-)

--

Saucerhead lingo #2102 "However, since PTP is in reality NOT a budding
astrophysicist..." ... "Perhaps if we try distraction as a tactic people
will forget we cannot answer simple conflicting issues with our nonsense
theory"

--
Posted via a free Usenet account from http://www.teranews.com

Yousuf Khan

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Jan 10, 2007, 8:50:23 PM1/10/07
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Sam Wormley wrote:
>> Wow, from this it looks like it's going to be orbitting at the L2
>> Lagrangian Point, which would mean that it is stationary with respect
>> to the Earth and the Sun, and it will be behind the Earth on the night
>> side of the planet at all times, 1.5 million km's out. If it's behind
>> the Earth, wouldn't it automatically be shielded from the Sun's rays?
>> Or is the Earth's shadow not big enough out there?
>>
>> Yousuf Khan
>
>
> The earth's moon will perturb it position.

Well, that's for sure, but I don't think it's going to be sitting
smack-dab in the centre of the Lagrangian point. I think it's actually
expected to "orbit" the point.

Also the Moon is 384 thousand km out, whereas the Lagrangian point is
1.5 million km out, so it's almost exactly 4 times further away than the
Moon. I don't think it's going to be a huge perturbation, and it will
likely be damped out by the orbiting motion.

Yousuf Khan

Sam Wormley

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Jan 10, 2007, 10:56:36 PM1/10/07
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Yousuf Khan

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Jan 10, 2007, 9:06:30 PM1/10/07
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AustinMN wrote:
> I can't answer your question, but from my understanding, being cold
> (i.e. shielded from the sun) is a good thing when studying infrared.
> Is it nuclear powered?

Actually, the webpage I linked to showed that it's got solar panels, and
in fact it's got something that no other space craft has ever had, a
huge sunshade which protects the telescope from the Sun. So obviously,
NASA thinks the Sun is still going to be visible from the L2 point. I
was just wondering out loud how big the Earth's shadow could be at that
distance?

Yousuf Khan

Sam Wormley

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Jan 10, 2007, 11:30:34 PM1/10/07
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Another craft alread in the L2 position used a sunshade--continuously
shaded from the Sun, Earth, and Moon by the spacecraft to allow lower
thermal disturbances.
http://map.gsfc.nasa.gov/about/aboutmap_pl.html


Length of Earth's shadow (umbra) is about 1,400,000 km.
L2 is about 1,500,000 km.

Timberwoof

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Jan 11, 2007, 12:09:32 AM1/11/07
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In article <45a55f27$1...@news.bnb-lp.com>, Yousuf Khan <bbb...@yahoo.com>
wrote:

http://en.wikipedia.org/wiki/Image:Lagrange_points.jpg

The math is easy: you need the size of the Sun, the distance to the
Earth, the size of the Earth, and the distance to L2. But it gets
complicated... The red arrows that point inward and the blue arrows that
point outward mean that the point is semistable. You can orbit around
the L2 (or L1 or L3) when the orbit is in a plane perpendicular to the
line between the Sun and Earth. The size of that orbit determines
whether the satellite sees the Sun.

(The Wikipedia article on Lagrange points is probably also worth
reading. I'm an expert because I read it. ;-)

--
Timberwoof <me at timberwoof dot com> http://www.timberwoof.com
It's easy to say a war is so important your neighbor should go fight it for you.

Henry Spencer

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Jan 10, 2007, 11:01:26 PM1/10/07
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In article <45a55f27$1...@news.bnb-lp.com>, Yousuf Khan <bbb...@yahoo.com> wrote:
>> http://en.wikipedia.org/wiki/James_Webb_Space_Telescope
>Wow, from this it looks like it's going to be orbitting at the L2
>Lagrangian Point, which would mean that it is stationary with respect to
>the Earth and the Sun, and it will be behind the Earth on the night side
>of the planet at all times, 1.5 million km's out. If it's behind the
>Earth, wouldn't it automatically be shielded from the Sun's rays? Or is
>the Earth's shadow not big enough out there?

L2 is too far out for Earth to block the Sun completely. There is some
blockage, which helps make the L2 region a good place for infrared
telescopes, but solar power is still practical there if you oversize
the solar arrays somewhat.
--
spsystems.net is temporarily off the air; | Henry Spencer
mail to henry at zoo.utoronto.ca instead. | he...@spsystems.net

mme...@cars3.uchicago.edu

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Jan 11, 2007, 12:41:53 AM1/11/07
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>distance.

Actually, since the distance of L2 is about 0.01au and the Earth's
diameter is about 0.01 of this of the Sun, L2 is in the vicinity of
the distance where the Earth's shadow dwindles to nothing.

Mati Meron | "When you argue with a fool,
me...@cars.uchicago.edu | chances are he is doing just the same"

Matthew Ota

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Jan 11, 2007, 12:46:26 AM1/11/07
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The JWST deployment video shows two sets of solar arrays deploying from
the instrument bus. The spacecraft in not nuclear powered.

Matthew Ota

Timberwoof

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Jan 11, 2007, 1:09:55 AM1/11/07
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In article <45a5baf0$1...@news.bnb-lp.com>, Yousuf Khan <bbb...@yahoo.com>
wrote:

Hmmm. Do you think geometry might supply the answer? How big is the
craft's orbit about L2?

WillE1

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Jan 11, 2007, 1:19:14 AM1/11/07
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"Yousuf Khan" <bbb...@yahoo.com> wrote in message
news:45a55f27$1...@news.bnb-lp.com...

Assuming objects sent to L2 don't bump into one another, here are a few more
L2 environmental concerns.
http://maxim.gsfc.nasa.gov/documents/Mission_Concept_Work/ISAL_January_2002_SST/SST_ISAL-1/Super_Star_Tracker/L2-natural-environment.pdf

Will E.


Robert Clark

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Jan 11, 2007, 10:05:03 AM1/11/07
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dbo...@mindspring.com wrote:
> AustinMN wrote:
> > Robert Clark wrote:
> > <snip>
> > > So going by this rate, it'll be 3mm/pixel 2.5 years after that, and
> > > 300 microns 1.25 years after that, and ...
> > > Hmm, in less than a decade then we should be able to resolve microbes
> > > from space.
> ...

> The required smoothness is no problem. I have a Be mirror flat that
> was polished to about 10 angstrom roughness.

How wide is this flat? Could it be polished to this smoothness for a
parabolic mirror?


Bob Clark

Robert Clark

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Jan 11, 2007, 11:30:28 AM1/11/07
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Robert Clark wrote:
> ...

> To resolve 3 cm in the optical from say a 300 km orbit would require a
> 6 meter mirror. The James Webb Space Telescope will have a 6.5 meter
> mirror and is scheduled for launch in 2013. But it was originally
> scheduled for launch in 2011:
>
> James Webb Space Telescope.
> http://en.wikipedia.org/wiki/James_Webb_Space_Telescope
>
> ...

The mirror of the JWST weighs less than 400 kg out of the total weight
of 6000 kg.
It would be interesting to find out how much a craft with this size
mirror would weigh if it didn't have to carry the sunshield and
associated mass for keeping the craft at cryogenic temperatures.

Bob Clark

Steve Willner

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Jan 11, 2007, 5:46:28 PM1/11/07
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[irrelevant groups snipped]

In article <Uziph.300832$FQ1.29488@attbi_s71>,


Sam Wormley <swor...@mchsi.com> writes:
> Here is what happens at L2 perturb by the moon...
> http://map.gsfc.nasa.gov/about/aboutmap_pl.html
> http://map.gsfc.nasa.gov/aboutmedia/990029.jpg

That diagram is not about perturbations at L2. It shows how WMAP
took advantage of gravitational assists from the Moon to _reach_ L2.
JWST could do the same thing, but I don't believe it's part of the
present mission plan. (The lunar assist orbit was a very clever
invention of some folks at GSFC.)

As others have written, JWST will be in a "halo orbit" around L2 and
will be in continuous sunlight. It needs several layers of sun
shielding to keep the telescope cold. Many previous missions have
used similar techniques. There's a diagram of the JWST configuration
at http://jwst.gsfc.nasa.gov/observatory.html .

I haven't explored the rest of the web site, but there are more
details in some of the links. I expect most of the relevant
information is there somewhere.

--
Steve Willner Phone 617-495-7123 swil...@cfa.harvard.edu
Cambridge, MA 02138 USA
(Please email your reply if you want to be sure I see it; include a
valid Reply-To address to receive an acknowledgement. Commercial
email may be sent to your ISP.)

Sam Wormley

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Jan 11, 2007, 6:03:26 PM1/11/07
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Steve Willner wrote:
> [irrelevant groups snipped]
>
> In article <Uziph.300832$FQ1.29488@attbi_s71>,
> Sam Wormley <swor...@mchsi.com> writes:
>> Here is what happens at L2 perturb by the moon...
>> http://map.gsfc.nasa.gov/about/aboutmap_pl.html
>> http://map.gsfc.nasa.gov/aboutmedia/990029.jpg
>
> That diagram is not about perturbations at L2. It shows how WMAP
> took advantage of gravitational assists from the Moon to _reach_ L2.
> JWST could do the same thing, but I don't believe it's part of the
> present mission plan. (The lunar assist orbit was a very clever
> invention of some folks at GSFC.)

Please note the continuing loop at the L2 position.

Robert Clark

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Jan 12, 2007, 9:24:52 AM1/12/07
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Sam Wormley wrote:

> Robert Clark wrote:
>
> > To resolve 3 cm in the optical from say a 300 km orbit would require a
> > 6 meter mirror.
>
> What is the effect of the Martian atmosphere on the goal of 3 cm
> resolution?

It is very likely less than for Earth's atmosphere which gives a best
resolution of .5 arcsec even from the best sites. A .5 arcsec
resolution at 300 km would correspond to a resolution of 11 cm on the
ground. In Mars thinner atmosphere, the results would probably be much
better than this.
However, I've seen some references that suggest the effects of air
scattering is much less pronounced looking down, as with spy
satellites, than when looking up, as with telescopes.
In any case adaptive optics could probably bring the seeing close to
the diffraction limit anyway.

Bob Clark

Sam Wormley

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Jan 12, 2007, 12:16:24 PM1/12/07
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Thanks Bob.

Steve Willner

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Jan 12, 2007, 12:21:23 PM1/12/07
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> >> http://map.gsfc.nasa.gov/about/aboutmap_pl.html

In article <2nzph.301886$FQ1.216029@attbi_s71>,


Sam Wormley <swor...@mchsi.com> writes:
> Please note the continuing loop at the L2 position.

Right. That's the halo orbit; nothing to do with lunar
perturbations.

Sam Wormley

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Jan 12, 2007, 12:25:44 PM1/12/07
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Steve Willner wrote:
>>>> http://map.gsfc.nasa.gov/about/aboutmap_pl.html
>
> In article <2nzph.301886$FQ1.216029@attbi_s71>,
> Sam Wormley <swor...@mchsi.com> writes:
>> Please note the continuing loop at the L2 position.
>
> Right. That's the halo orbit; nothing to do with lunar
> perturbations.
>

And the period of the halo orbit is not in sync with
the lunar period?

Steve Willner

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Jan 15, 2007, 12:40:33 PM1/15/07
to
In article <swPph.302917$FQ1.183562@attbi_s71>,

Sam Wormley <swor...@mchsi.com> writes:
> And the period of the halo orbit is not in sync with
> the lunar period?

Correct. See http://www.stsci.edu/jwst/overview/design/orbit.html
Halo orbit period is about 6 months. I think it must depend on the
radius of the orbit around L2; presumably one could select a period
of one lunar month, though it would seem to be a bad idea. Another
paper that discusses the concepts is at
http://highorbits.jhuapl.edu/missions.htm , and the Wikipedia article
at http://en.wikipedia.org/wiki/Lagrangian_point looks pretty good,
though I haven't studied it in detail.

You can guess that lunar perturbations will be negligible just from
noticing how far L2 is from lunar orbit. Perturbations will be
roughly (1/81)*(D_moon/D_earth)^2, where 1/81 is the Moon/Earth mass
ratio and D_moon/D_earth is the radius of the Moon's orbit divided by
the distance from Earth to L2.

Sam Wormley

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Jan 15, 2007, 1:06:04 PM1/15/07
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Steve Willner wrote:
> In article <swPph.302917$FQ1.183562@attbi_s71>,
> Sam Wormley <swor...@mchsi.com> writes:
>> And the period of the halo orbit is not in sync with
>> the lunar period?
>
> Correct. See http://www.stsci.edu/jwst/overview/design/orbit.html
> Halo orbit period is about 6 months. I think it must depend on the
> radius of the orbit around L2; presumably one could select a period
> of one lunar month, though it would seem to be a bad idea. Another
> paper that discusses the concepts is at
> http://highorbits.jhuapl.edu/missions.htm , and the Wikipedia article
> at http://en.wikipedia.org/wiki/Lagrangian_point looks pretty good,
> though I haven't studied it in detail.
>
> You can guess that lunar perturbations will be negligible just from
> noticing how far L2 is from lunar orbit. Perturbations will be
> roughly (1/81)*(D_moon/D_earth)^2, where 1/81 is the Moon/Earth mass
> ratio and D_moon/D_earth is the radius of the Moon's orbit divided by
> the distance from Earth to L2.
>

Thank you, Steve.
-Sam

Keith F. Lynch

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Jan 15, 2007, 10:31:38 PM1/15/07
to

>> Wow, from this it looks like it's going to be orbitting at the

>> L2 Lagrangian Point, ...

> L2 is too far out for Earth to block the Sun completely. There
> is some blockage, which helps make the L2 region a good place for
> infrared telescopes, but solar power is still practical there if
> you oversize the solar arrays somewhat.

Presumably the solar panels will be flat, shiny, and perpendicular to
the direction to the sun -- which is also the direction to the earth.
Will it show up as a bright spark of light in the night sky, a
perpetual sun glint?
--
Keith F. Lynch - http://keithlynch.net/
Please see http://keithlynch.net/email.html before emailing me.

bbbl67

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Jan 16, 2007, 1:03:33 AM1/16/07
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On Jan 10, 11:30 pm, Sam Wormley <sworml...@mchsi.com> wrote:
> Length of Earth's shadow (umbra) is about 1,400,000 km.
> L2 is about 1,500,000 km.

Ah, I see, thanks.

bbbl67

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Jan 16, 2007, 1:18:06 AM1/16/07
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On Jan 11, 12:41 am, mme...@cars3.uchicago.edu wrote:
> Actually, since the distance of L2 is about 0.01au and the Earth's
> diameter is about 0.01 of this of the Sun, L2 is in the vicinity of
> the distance where the Earth's shadow dwindles to nothing.

I assume from that if the Earth were in orbit around a star like VY
Canis Majoris (2000x diameter of the Sun), we can forget about having a
much of a night-time, as the light from it is likely going to wrap
around the far side of the planet almost? :-)

Yousuf Khan

bbbl67

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Jan 16, 2007, 1:53:09 AM1/16/07
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On Jan 15, 10:31 pm, "Keith F. Lynch" <k...@KeithLynch.net> wrote:
> Presumably the solar panels will be flat, shiny, and perpendicular to
> the direction to the sun -- which is also the direction to the earth.
> Will it show up as a bright spark of light in the night sky, a
> perpetual sun glint?

At 1.5 million miles away, I doubt it will even be seen without a
telescope.

Yousuf Khan

Oren Tirosh

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Jan 16, 2007, 5:31:03 AM1/16/07
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Henry Spencer wrote:

> In article <45a55f27$1...@news.bnb-lp.com>, Yousuf Khan <bbb...@yahoo.com> wrote:
> >> http://en.wikipedia.org/wiki/James_Webb_Space_Telescope
> >Wow, from this it looks like it's going to be orbitting at the L2
> >Lagrangian Point, which would mean that it is stationary with respect to
> >the Earth and the Sun, and it will be behind the Earth on the night side
> >of the planet at all times, 1.5 million km's out. If it's behind the
> >Earth, wouldn't it automatically be shielded from the Sun's rays? Or is
> >the Earth's shadow not big enough out there?
>
> L2 is too far out for Earth to block the Sun completely. There is some
> blockage, which helps make the L2 region a good place for infrared
> telescopes, but solar power is still practical there if you oversize
> the solar arrays somewhat.

Does any planet have L2 within its cone of darkness? Jupiter perhaps?

Robert Clark

unread,
Jan 16, 2007, 10:26:27 AM1/16/07
to

Actually you should check this calculation. I'm informed via email of
a mistake I made in calculating resolution on a different topic. I
didn't convert from degrees to radians.
Let me try that again. The formula for resolution is the Rayleigh
criterion:

Angular resolution.
http://en.wikipedia.org/wiki/Angular_resolution#Explanation

sin(θ) = 1.22λ/D

Now for the angle θ small and in *radians*, sin(θ) is approximately
equal to θ, and θ is approximately equal to the ratio of the distance
to be resolved divided by the distance to the object. A .5 arcsec angle
is .5/(60*60) = 1/7200 degrees. Converting to radians this is
(2π/360)/7200 radians. Then this is about the ratio of the resolvable
distance to the distance to the body. At 300,000 m then the resolvable
distance would be 300,000 times this or .73 m, 73 cm.
But note this is assuming the atmospheric dispersion puts the same
limit on resolution looking down as with looking up. As I said there is
some question about this. And in the case for Mars the atmospheric
dispersion limits should also be much less.


Bob Clark

mme...@cars3.uchicago.edu

unread,
Jan 16, 2007, 12:51:41 PM1/16/07
to
> sin(=CE=B8) =3D 1.22=CE=BB/D
>
> Now for the angle =CE=B8 small and in *radians*, sin(=CE=B8) is approximat=
>ely
>equal to =CE=B8, and =CE=B8 is approximately equal to the ratio of the dist=

>ance
>to be resolved divided by the distance to the object. A .5 arcsec angle
>is .5/(60*60) =3D 1/7200 degrees. Converting to radians this is
>(2=CF=80/360)/7200 radians. Then this is about the ratio of the resolvable

>distance to the distance to the body. At 300,000 m then the resolvable
>distance would be 300,000 times this or .73 m, 73 cm.
> But note this is assuming the atmospheric dispersion puts the same
>limit on resolution looking down as with looking up.

Where do you see ***anything*** having to do with atmospheric
dispersion in the above.


As I said there is
>some question about this. And in the case for Mars the atmospheric
>dispersion limits should also be much less.

Robert Clark

unread,
Jan 16, 2007, 2:44:07 PM1/16/07
to

When I was giving the resolution a mirror about Mars could achieve I
was assuming it would give diffraction limited performance. Sam raised
the question about the additional limits imposed by atmospheric
scattering.
On Earth for telescopes looking up, this puts a limit on the
resolution a scope can achieve no matter what its size. The number I've
seen quoted was the best you could do would be no better than that of a
20" scope regardless of the telescope size(!)
Large ground based telescopes without methods to counteract this such
as adaptive optics can only detect fainter objects not achieve better
resolution.
However, I've seen mentioned this doesn't happen for a satellite
looking down to the surface. In that case resolution limits imposed by
the atmosphere wouldn't apply for satellites imaging the surface.
There was some discussion on this topic on this thread:

Spy satellites for astronomy.
http://groups.google.com/group/sci.astro/browse_frm/thread/a0ae7386d655847c/


Bob Clark

mme...@cars3.uchicago.edu

unread,
Jan 16, 2007, 3:31:52 PM1/16/07
to

The sentence you wrote above, following the discussion of resolution
limits, says "this is assuming the atmospheric dispersion puts the same
limit on resolution looking down as with looking up.". Well, no, if
"this" refers to the text quoted then *this* is assuming nothing of the
sort. That's all.

zzbu...@netscape.net

unread,
Jan 18, 2007, 10:42:22 PM1/18/07
to

Sam Wormley wrote:
> Robert Clark wrote:
>
> > To resolve 3 cm in the optical from say a 300 km orbit would require a
> > 6 meter mirror.
>
> What is the effect of the Martian atmosphere on the goal of 3 cm
> resolution?

None, since orbital resolution is not how
you resolve microbes.
The only thing sattelites are going to
determine is the dust concentration in
the Martian atmoshere.

Keith F. Lynch

unread,
Jan 20, 2007, 2:36:32 PM1/20/07
to

I think you mean 1.5 million *kilometers*.

A specular reflector reflecting the sun is *much* brighter than a
white object. The sun's disk is about 9.3 milliradians wide in
Earth's sky, and has an apparent magnitude of -26.7. A specular
reflection of the sun, from a perfect reflector, will have the same
amount of light per unit solid angle.

Five magnitudes represent a factor of a hundred. An object with one
tenth the angular diameter will reflect one hundredth the light,
so will be five magnitudes dimmer. At 1.5 million kilometers, a
reflector 14,000 kilometers wide will appear as bright as the sun (if
it's aimed just right). Knock it down to 14 kilometers, and that
drops it by 15 magnitudes, to -11.73, still bright enough to read
by. Knock it down to 14 meters, and you're down to +3.3, still a
moderately bright star.

At L2, the Earth will block 84% of the sunlight, so make it +5.3. And
of course the solar panels aren't perfect reflectors, and probably
aren't 14 meters in diameter. But the final number is intriguingly
close to the limit of the normal unaided eye (+6.0). Even if it isn't
quite visible to the eye, it ought to be easily visible in a pair of
basic binoculars.

But a better question is why the successor to the 17-year-old Hubble
Space Telescope is intended to last only five years. And cannot be
reached for maintenance. And will be unable to do some things Hubble
could do.

bbbl67

unread,
Jan 20, 2007, 4:35:07 PM1/20/07
to
On Jan 20, 2:36 pm, "Keith F. Lynch" <k...@KeithLynch.net> wrote:
> But a better question is why the successor to the 17-year-old Hubble
> Space Telescope is intended to last only five years. And cannot be
> reached for maintenance. And will be unable to do some things Hubble
> could do.

Well how long was Hubble originally supposed to last? I seem to recall
it's gone a little longer than its originally anticipated expiration
date. It's likely JW might go longer than its expiration date too.

Also there was some talk of outfitting Hubble with some of the new
robot arms that the space station has, so that it could do some
self-servicing. That was one of the proposals in lieu of sending a
space shuttle to service it. I wonder why they didn't outfit the JW
with some of these robot arms at the outset?

Also it will take 5 seconds for signals to reach back and forth from
Earth to the L2 point.

Yousuf Khan

Keith F. Lynch

unread,
Jan 20, 2007, 6:12:39 PM1/20/07
to
bbbl67 <yjk...@gmail.com> wrote:
> Well how long was Hubble originally supposed to last?

I don't know. Do you? I assumed it was intended to last indefinitely
with routine maintenance, just like observatories on the ground.

When the 200-inch Hale Telescope was completed in 1948, there were no
plans to shut it down in 1953, or even 1965. It is of course still in
operation, as are telescopes more than twice as old.

> Also there was some talk of outfitting Hubble with some of the new
> robot arms that the space station has, so that it could do some
> self-servicing. That was one of the proposals in lieu of sending a
> space shuttle to service it. I wonder why they didn't outfit the JW
> with some of these robot arms at the outset?

Because they are incapable of doing much? Human hands, even in
spacesuit gloves, are still incomparably more dextrous. Besides,
sometimes spare parts are needed, such as the replacement gyros and
batteries now needed at Hubble.

William Hamblen

unread,
Jan 20, 2007, 8:58:24 PM1/20/07
to
On 2007-01-20, Keith F. Lynch <k...@KeithLynch.net> wrote:
> bbbl67 <yjk...@gmail.com> wrote:
>> Well how long was Hubble originally supposed to last?
>
> I don't know. Do you? I assumed it was intended to last indefinitely
> with routine maintenance, just like observatories on the ground.

That isn't a good assumptiom, because maintaining the Space
Telescope is far more difficult than maintaining a ground based
telescope. The original mission planning was for a service life
of 15 years.

Bud

Joseph Lazio

unread,
Jan 22, 2007, 7:09:12 AM1/22/07
to
>>>>> "KFL" == Keith F Lynch <k...@KeithLynch.net> writes:

KFL> But a better question is why the successor to the 17-year-old
KFL> Hubble Space Telescope is intended to last only five years.

You need to distinguish between planned and operational lifetimes.
The Mars rovers had planned lifetimes of 90 days; three years later
they are still going. The Voyager spacecraft have had their lifetimes
extended several times.

The more important question to ask is the following: What would you
give up in order to have JWST last for 10 years? Doubling the mission
lifetime could potentially double the price tag for the telescope.
That means that some other mission might not fly. Which proposed
missions would you defer/cancel?

Note that this is not simply an academic exercise. The Voyager
Interstellar Missions have narrowly escaped being turned off on a
couple of occasions. Even on the ground, the National Science
Foundation is seriously considering a recommendation to shut down
fully-functional, operational telescopes in an effort to bring new
ones online.

KFL> And cannot be reached for maintenance.

That's not clear. There's been some discussion of whether equipment
developed as part of the Vision for Space Exploration might also
enable the repair of telescopes at L2.

Also, see cost, above.

KFL> And will be unable to do some things Hubble could do.

What's the point? Hubble's been there, done that. The point of new
telescopes is not to be able to do things that previous ones have
done, but to do what previous ones could not.

--
Lt. Lazio, HTML police | e-mail: jla...@patriot.net
No means no, stop rape. | http://patriot.net/%7Ejlazio/
sci.astro FAQ at http://sciastro.astronomy.net/sci.astro.html

Keith F. Lynch

unread,
Jan 22, 2007, 8:18:04 PM1/22/07
to
Joseph Lazio <jla...@adams.patriot.net> wrote:
> What's the point? Hubble's been there, done that. The point of new
> telescopes is not to be able to do things that previous ones have
> done, but to do what previous ones could not.

Telescopes aren't like synchrotrons. Once you've seen what happens
when you smash protons together at 100 GeV, there's nothing left
to do except build a 200 GeV synchrotron and try again. But with
a telescope, it probably hasn't yet been pointed in all possible
directions. And even if it has, there are always new things going on
in the sky -- novas, comets, new storms on Jupiter, GRBs, you name it.
That's why hundred-year-old telescopes are still in use, and are still
doing cutting-edge science.

Joseph Lazio

unread,
Jan 24, 2007, 6:57:33 AM1/24/07
to
>>>>> "KFL" == Keith F Lynch <k...@KeithLynch.net> writes:

KFL> Joseph Lazio <jla...@adams.patriot.net> wrote:
>> What's the point? Hubble's been there, done that. The point of
>> new telescopes is not to be able to do things that previous ones
>> have done, but to do what previous ones could not.

KFL> Telescopes aren't like synchrotrons. Once you've seen what
KFL> happens when you smash protons together at 100 GeV, there's
KFL> nothing left to do except build a 200 GeV synchrotron and try
KFL> again. But with a telescope, it probably hasn't yet been pointed
KFL> in all possible directions. And even if it has, there are always
KFL> new things going on in the sky -- novas, comets, new storms on
KFL> Jupiter, GRBs, you name it. That's why hundred-year-old
KFL> telescopes are still in use, and are still doing cutting-edge
KFL> science.

I'm certainly receptive to arguments about a dynamic sky (as I'm
serving on the scientific organizing committee for a meeting focussed
on that). However, I'm not sure that your arguments make the best
case for Hubble.

If I want to study the dynamic sky, why should I prefer Hubble (or a
son-of-Hubble) to a smaller, wide-field instrument that could scan the
sky quickly (see LSST)? (Actually, LSST will have a wider field, but
won't be any smaller than Hubble.)

If I want to study Jupiter, why not use the same amount of money to
send an orbiter to Jupiter?

If I want to study comets, why not fund a half dozen missions similar
to Deep Impact?

Moreover, what about the science that Hubble cannot do? The JWST is
being designed, in part, to search out the first stars in the
Universe. That requires an IR-optimized telescope, something Hubble
is not. JWST might still be able to do some of the things that you
describe, yet also (it is hoped) be able to find the first stars.
Isn't that also exciting?

Keith F. Lynch

unread,
Jan 24, 2007, 11:06:30 PM1/24/07
to
Joseph Lazio <jla...@adams.patriot.net> wrote:
> I'm certainly receptive to arguments about a dynamic sky (as I'm
> serving on the scientific organizing committee for a meeting
> focussed on that). However, I'm not sure that your arguments make
> the best case for Hubble.

> If I want to study the dynamic sky, why should I prefer Hubble (or a
> son-of-Hubble) to a smaller, wide-field instrument that could scan
> the sky quickly (see LSST)?

To see smaller, fainter, or more distant objects, of course.

> If I want to study Jupiter, why not use the same amount of money to
> send an orbiter to Jupiter?

A powerful telescope in Earth orbit can study *all* the planets,
not just the one it's next to. And can also study comets, moons,
asteroids, Kuiper belt objects, stars, pulsars, nebulas, galaxies,
quasars, and GRBs, too.

Besides, if something suddenly happens on Jupiter, it's a lot faster
to aim an existing telescope at it than to design, build, and launch a
probe, and wait for it to arrive at Jupiter.

> Moreover, what about the science that Hubble cannot do? The JWST
> is being designed, in part, to search out the first stars in the
> Universe. That requires an IR-optimized telescope, something Hubble
> is not. JWST might still be able to do some of the things that you
> describe, yet also (it is hoped) be able to find the first stars.
> Isn't that also exciting?

Exciting enough that it shouldn't be thrown away after five years.

Speaking of the beginnings of things and the dynamic sky, just how
precisely can the absolute -- not relative -- microwave background be
measured? Is there any chance that it can be measured so precisely
that two such measurements, perhaps a decade apart, can actually
directly detect the cooling and expansion of the universe? Am I
correct in assuming the cooling is roughly proportional to the
age of the universe, about one part in a billion in a decade?

Steve Willner

unread,
Jan 25, 2007, 4:10:43 PM1/25/07
to
In article <ep9ac6$l4s$1...@panix3.panix.com>,

"Keith F. Lynch" <k...@KeithLynch.net> writes:
> Exciting enough that it shouldn't be thrown away after five years.

One thing HST has demonstrated, alas, is that servicing is far more
expensive than launching a new telescope every few years. That's
ignoring fixed costs of the shuttle; servicing is even more expensive
if you include those.

Another concern is the factor of three or so inefficiency of
operating a telescope in LEO. Worse yet, because of thermal
problems, I don't see how it would be possible to build a
JWST-equivalent that could operate at all in any orbit the shuttle
could reach. How do you shield the telescope from both Sun and Earth
at all times and positions in the orbit?

> Speaking of the beginnings of things and the dynamic sky, just how
> precisely can the absolute -- not relative -- microwave background be
> measured?

Fixsen & Mather (2002 ApJ 581, 817) write in their abstract, "The
measurement of the deviation of the CMB spectrum from a
2.725+/-0.001 K blackbody form made by the COBE-FIRAS could be
improved by nearly 2 orders of magnitude."

Measuring the temperature itself rather than the deviations should be
easier, but a part in 10^9 looks pretty far away. However, I haven't
read the paper cited above, let alone other ones, so perhaps there's
something I'm missing. Good idea if it could be made to work.

Greg Hennessy

unread,
Jan 25, 2007, 11:57:29 PM1/25/07
to
On 2007-01-25, Steve Willner <wil...@cfa.harvard.edu> wrote:
> One thing HST has demonstrated, alas, is that servicing is far more
> expensive than launching a new telescope every few years. That's
> ignoring fixed costs of the shuttle; servicing is even more expensive
> if you include those.

I'm not sure that is true. I don't think it is clear which is more
expensive, building a new instrument every 5 years or so, or launching
a new telescope every five years. I doubt there would be funding for a
new hubble class mission every 5 years.

Steve Willner

unread,
Jan 26, 2007, 1:11:53 PM1/26/07
to
In article <slrnerj2hq.ohs...@localhost.localdomain>,

Greg Hennessy <greg.h...@localhost.localdomain> writes:
> I don't think it is clear which is more
> expensive, building a new instrument every 5 years or so, or launching
> a new telescope every five years.

Building the new instruments is a minor part of the servicing cost.

> I doubt there would be funding for a new hubble class mission every
> 5 years.

That's a reasonable concern, though unless JWST has another massive
overrun, I don't think we'll see a "Hubble cost" mission ever again.

Greg Hennessy

unread,
Jan 26, 2007, 9:50:09 PM1/26/07
to
On 2007-01-26, Steve Willner <wil...@cfa.harvard.edu> wrote:
> In article <slrnerj2hq.ohs...@localhost.localdomain>,
> Greg Hennessy <greg.h...@localhost.localdomain> writes:
>> I don't think it is clear which is more
>> expensive, building a new instrument every 5 years or so, or launching
>> a new telescope every five years.
>
> Building the new instruments is a minor part of the servicing cost.

Of course. But launching a new instrument is about a factor of two of
a billion dollars. Launching a repair mission is about a factor of two
of a billion dollars. Of course with the shuttle going away, we won't
have the option of repair missions anymore.

> That's a reasonable concern, though unless JWST has another massive
> overrun, I don't think we'll see a "Hubble cost" mission ever again.

Has JWST ever NOT had a massive cost overrun?


Joseph Lazio

unread,
Jan 27, 2007, 2:13:56 PM1/27/07
to
>>>>> "KFL" == Keith F Lynch <k...@KeithLynch.net> writes:

KFL> Joseph Lazio <jla...@adams.patriot.net> wrote:
>> I'm certainly receptive to arguments about a dynamic sky (...).


>> However, I'm not sure that your arguments make the best case for
>> Hubble.

>> If I want to study the dynamic sky, why should I prefer Hubble

>> (...) to a smaller, wide-field instrument that could scan the sky
>> quickly (see LSST)?

KFL> To see smaller, fainter, or more distant objects, of course.

Field of view is often far more important than sensitivity. With the
Hubble's field of view, it would take about 1 year to survey the
entire sky, if one spent only 1 second for each pointing. The LSST
will cover the entire sky in 3 nights. Also, the LSST will have an
8.4 meter aperture, compared to Hubble's 2.4 meter aperture.

Surprisingly, distance is often unimportant. One of the key targets
for dynamic sky observations is afterglows from gamma-ray bursts.
These are coming from half-way across the observable Universe, but one
doesn't know in which direction. Thus, it is more important to be
able to survey the entire sky quickly.


>> If I want to study Jupiter, why not use the same amount of money to
>> send an orbiter to Jupiter?

KFL> A powerful telescope in Earth orbit can study *all* the planets,
KFL> not just the one it's next to. And can also study comets, moons,
KFL> asteroids, Kuiper belt objects, stars, pulsars, nebulas,
KFL> galaxies, quasars, and GRBs, too.

KFL> Besides, if something suddenly happens on Jupiter, it's a lot
KFL> faster to aim an existing telescope at it than to design, build,
KFL> and launch a probe, and wait for it to arrive at Jupiter.

Yes, but these are the kinds of arguments that one has to use. Is it
better to spend $1 billion for a telescope around Earth, or $0.2
billion to produce 5 probes to different parts of the solar system?

>> Moreover, what about the science that Hubble cannot do? The JWST
>> is being designed, in part, to search out the first stars in the
>> Universe. That requires an IR-optimized telescope, something
>> Hubble is not. JWST might still be able to do some of the things
>> that you describe, yet also (it is hoped) be able to find the first
>> stars. Isn't that also exciting?

KFL> Exciting enough that it shouldn't be thrown away after five
KFL> years.

Which brings us back to the cost issue that I raised in a previous
message. If the operational lifetime of the JWST is extended, that
means something else cannot be done. What would that something else
be?

Similarly, it is now apparent that keeping Hubble going is causing
other missions to be canceled. Is that the best use of the money?

Joseph Lazio

unread,
Jan 27, 2007, 2:26:46 PM1/27/07
to
>>>>> "KFL" == Keith F Lynch <k...@KeithLynch.net> writes:

KFL> Speaking of the beginnings of things and the dynamic sky, just
KFL> how precisely can the absolute -- not relative -- microwave
KFL> background be measured? Is there any chance that it can be
KFL> measured so precisely that two such measurements, perhaps a
KFL> decade apart, can actually directly detect the cooling and
KFL> expansion of the universe? Am I correct in assuming the cooling
KFL> is roughly proportional to the age of the universe, about one
KFL> part in a billion in a decade?

This isn't quite what you asked, but a better way of approach this
question would be to look for the change of an object's redshift over
time. This is known as the Sandage test or the Sandage-Loeb test.

Over the course of a decade, the redshift of an object at z ~ 1 should
change by about 1E-9. This change is too small to see for a given
absorption line. However, a typical quasar spectrum might have
hundreds of absorption lines in it. If one could observe the
absorption lines of about 100 quasars, then statistically one might be
able to detect this change, which would amount to a direct detection
of the expansion of the Universe.

See Corasaniti et al. (2007, <URL:
http://xxx.lanl.gov/abs/astro-ph/0701433 >) for discussion, references,
and more details.

Steve Willner

unread,
Jan 29, 2007, 7:19:00 PM1/29/07
to
In article <slrnerlff1.3vg...@localhost.localdomain>,

Greg Hennessy <greg.h...@localhost.localdomain> writes:
> But launching a new instrument is about a factor of two of
> a billion dollars.

Depends on how ambitious you are. If I'm not mistaken, two of the
four Great Observatories came in under $1G. (I know one of them did
and still would be under that figure even in today's dollars.)
Numerous smaller observatories, with a range of costs, are in orbit
now or in development. I think "Small Explorers" are still $25M, and
cheaper missions have flown.

> Launching a repair mission is about a factor of two of a billion
> dollars.

Barely, and only if you take the cost of a shuttle launch at the
smallest reasonable level. (NASA accounting for manned space flight
isn't meant to be transparent.) The biggest cost of servicing is
maintaining the "standing army" needed for servicing missions. Last
time I checked, that was about $300M/year for HST. Launch costs and
new instruments are not included, nor is the initial extra cost of
making the observatory serviceable in the first place.

As I wrote earlier, one thing HST has proved is that (financially, at
least) servicing is a bad idea.

> Of course with the shuttle going away, we won't
> have the option of repair missions anymore.

This, of course, pretty much settles any argument for near-future
missions.

> Has JWST ever NOT had a massive cost overrun?

Strange question. JWST has had one major cost overrun that I know
of, associated with the first realistic cost estimate for the
mission. (You might well ask why the first realistic estimate came
so late in the program.) Whether it will have further major overruns
is not clear to me. The fact that the same institutions that gave us
HST are managing JWST may be significant.

Greg Hennessy

unread,
Jan 29, 2007, 11:30:58 PM1/29/07
to
>> But launching a new instrument is about a factor of two of
>> a billion dollars.
>
> Depends on how ambitious you are. If I'm not mistaken, two of the
> four Great Observatories came in under $1G.

Yes, but they were also downscoped quite a bit to fit in the cost cap.

> Numerous smaller observatories, with a range of costs, are in orbit
> now or i